Aneuploidies, or abnormal numbers of chromosomes in a cell, originate from errors in segregation of chromosomes during the two highly regulated programs of cell division, mitosis and meiosis. The consequences of aneuploidies of meiotic origins are devastating as they contribute to 35% of clinically recognized miscarriages, 4% of still births, 20% of all birth defects and to virtually all germ cell tumors. There is also a clear environmental contribution in the etiology of meiotic chromosome segregation errors as compounds as diverse as plastics, pesticides and anti-anxiety drugs have been shown to cause meiotic aneuploidies in mammals. Despite the severe health outcomes of meiotic aneuploidies, the screening of environmental toxicants for their ability to disrupt meiosis and cause aneuploidy has been severely lagging. This problem is inherent to the complexity of the meiotic program which differs in part significantly from mitosis, cannot be efficiently recapitulated in vitro and initiates early on during mammalian development within the confines of the embryonic gonad. Thus, we currently lack the tools to efficiently and comprehensively interrogate our chemical environment for its effect on meiotically-derived aneuploidies. In this application, we propose the development a high-throughput screening approach that makes use of the remarkable biological features of the nematode Caenorhabditis elegans, which as a meiotic model system shows a high degree of conservation with humans. To this aim, we are building on a strategy developed for the isolation of mutants defective in chromosome segregation during meiosis. In the K99 section of the project, we will complete the changes in the design of the C. elegans transgenic strain used in this strategy to make it suitable for toxicant screening. These changes include a multi-assayable luciferase/GFP reporter system and an increased sensitivity by improving chemical penetration. We will test the validity of our approach by performing two small scale screens against compounds with known aneugenic activity and reproductive effects: one set of chemotherapeutic agents from the NCI and one of environmental toxicants from the EPA. In the second phase (R00) of the project, we will move our system to a large scale, high- throughput setting to screen two libraries, first the Toxcast Phase I 309 chemicals and then the Tox21 library of about 10,000 compounds. Secondary validation first in C. elegans and in mammalian models will conclusively establish the validity of the screen and likely lead to the identification of novel meiotic aneugenic compounds. The collaborative basis of this project which combines our expertise with that of key researchers at EPA and NIEHS and the tremendous training environment make us confident of the success of the approach. Together, we will be able, for the first time, to assess the meiotic aneugenic activity of our environment and improve our ability to predict toxicity to mammals and humans.